Power transmission shaft used for many machine components of automobiles, various industrial machines, and the like performs torque transmission normally by engaging serrations, splines, and the like formed on an outer periphery thereof with a mating member (boss). In consideration of plastic workability, machine workability, and cost, a torque-transmission teeth portion such as serrations or splines is used with higher axial strength obtained by subjecting medium carbon steel or low-alloy steel to surface-hardening treatment such as induction hardening, carburizing and quenching, or nitriding, or to heat treatment such as thermal refining.
For example, general manufacturing steps of a power transmission shaft used as a drive shaft for automobiles includes, in the following order, a cutting-out step illustrated in FIG. 5A of a bar-like base material, an outer diameter lathing step illustrated in FIG. 5B of the base material, a rolling step illustrated in FIG. 5C of serrations 5 at both end portions, a lathing step illustrated in FIG. 5D of clip grooves 6 at both the end portions, an induction hardening step and a induction tempering step illustrated in FIG. 5E, and an outer diameter bake-coating step illustrated in FIG. 5F.
As an example of a method of enhancing axial strength through heat treatment, there may be provided an invention described in Patent Literature 1. In the invention, a shaft-like machine component undergoes induction hardening or carburizing and quenching, and a ratio of an effective hardened-layer depth to a component radius is set to range from 0.4 to 0.8.
Further, each of Patent Literatures 2, 3, and 4 discloses an invention of setting the ratio of the effective hardened-layer depth to the component radius (hereinafter, referred to as hardened-layer ratio). The hardened-layer ratios of the inventions described in Patent Literatures 2, 3, and 4 are set to 0.4 or more, 0.45 or more, and 0.5 or more, respectively.
In the inventions described in the publications just described above, the torque-transmission teeth portion such as splines and the smooth-surface like smooth portion free from teeth of this type undergo heat treatment without particular distinction. In those inventions, the torque-transmission teeth portion and the smooth portion undergo heat treatment probably at the same time and under the same condition, and hardening depths in this case are substantially uniform over the torque-transmission teeth portion and the smooth portion (refer to hatched portion of FIG. 5E)
For enhancement of strength of the power transmission shaft, enhancement is required from both of the following aspects: static torsional strength (static strength) and torsional fatigue strength (dynamic strength). The static torsional strength is determined mainly based on a size of an axial diameter, and the fatigue strength is determined based on the axial diameter and a stress-concentration factor. Further, in comparison with static torsional strength of the torque-transmission teeth portion such as splines and that of the smooth portion, the static torsional strength of the smooth portion is lower than that of the torque-transmission teeth portion. Meanwhile, torsional fatigue strength of the smooth portion is apt to be higher than that of the torque-transmission teeth portion.
With a focus on difference in characteristics between the torque-transmission teeth portion and the smooth portion, in the inventions described in Patent Literatures 5 and 6, the torque-transmission teeth portion and the smooth portion are different from each other in hardening depth. That is, in the invention of Patent Literature 5 or 6, an effective hardened-layer depth (or hardened-layer ratio) of the smooth portion is set to be larger than an effective hardened-layer depth (or hardened-layer ratio) of the torque-transmission teeth portion.